It is well known that sheet-fed rotary offset printing machines subject the sheets to substantial tension; this is particularly so in five-cylinder systems, in which two blanket cylinders cooperate with a counter or impression printing cylinder. The tension applied to the sheet occurs during the time when the sheet is carried between the printing cylinder and the rubber blanket cylinders. These tension forces have the tendency to pull the sheet out of the grippers on the printing cylinder in which the sheet is held.
It has been observed that the forces which act on the sheet, and which may cause shifting of the upper leading edge thereon, may result in ghosts, and double-printing on the sheet, particularly along the trailing portion thereof. Even if the gripper closing forces are comparatively high, the grippers and the counter-engagement surface, or both, are roughened or knurled or otherwise made to have a high friction, ghosts on the sheet can still not be avoided at all times, since the sheet still may move or slip between the grippers due to the high tension forces acting on the sheets. The slip of the sheet in the grippers is frequently non-uniform across the width of the machine. Often, the sheet slips in the center region more than at the edges.
It has been proposed--see German Pat. No. 155,403--to apply projecting pins on the grippers which perforate the sheet to be gripped, in order to improve the reliability of holding of the sheet by the grippers. It has been found that such gripper systems in which pins are located on the grippers will improve the gripping effect only at specific points where the pins are located since, because the grippers are spaced from each other, not all of the grippers may close reliably.